Plasma is used in many industrial processes including semiconductor fabrication. Semiconductor plasma processing typically involves placing a wafer into a plasma processing chamber and exposing the wafer to a plasma for a specific period of time in achieve the desired process. Plasma processing steps commonly used in semiconductor integrated circuit fabrication include, deposition, etching and cleaning. A specific series of plasma processing steps are required to fabricate integrated circuits on a substrate each of which must be properly performed to produce functional integrated circuits on the wafer.
A single wafer may contain thousands of integrated circuits. Thus, when the plasma processing steps are properly performed, a high percentage of functional integrated circuits are produced on the semiconductor wafer. Errors in plasma processing result in defective integrated circuits and a lower percentage of functional integrated circuits are produced on a wafer.
Plasma processing errors may be caused by failing to stop the plasma process at the correct time or at the correct “end point” of the plasma process. For example, in a plasma etching process, two layers of material on the wafer are etched through and the etch end point is ideally immediately after the second layer of material has been etched through. If the etch process is stopped too early, the etch process will not be complete because the second layer is not be completely etched through, leaving excess material at the bottom of the etch. This excess material may cause the associated integrated circuit formed on the wafer to be defective and non-functional. If the plasma etch process continues beyond the etch end point, the underlying layer below the etch layer(s) is also etched. Because devices are formed with very thin layers of material, excessive etching can very easily etch completely through the underlying third layer which may also create a defective and non-functional integrated circuits on the wafer.
To reduce plasma processing errors, the plasma may be monitored so that the end point of a plasma process may be detected. When the end point is detected a control signal may be sent to the processing chamber to stop the plasma process. In some applications, the end point can be detected by monitoring the optical emissions of the plasma.